Moving blades of gas turbines are exposed to high temperatures and strong mechanical loads during operation. Nickel-based superalloys, which can be strengthened by precipitation of a γ′ phase, are therefore used with preference for such components. Nevertheless, cracks may occur over time in the moving blades and spread further as time progresses. Such cracks may be caused for instance by extreme mechanical loading during the operation of a gas turbine, but they may also already occur during the production process. Since the production of turbine blades and other workpieces of such superalloys is complex and cost-intensive, there are efforts to produce as little scrap as possible in production and to ensure a long service life of the products produced.
Gas turbine blades that are in operation are routinely serviced and exchanged where necessary, if satisfactory functioning can no longer be ensured with certainty because of operationally related loading. To make it possible for exchanged turbine blades to be used further, wherever possible they are refurbished. They can then be used once again in a gas turbine. Such refurbishment may, for example, involve the necessity for build-up welding in damaged regions, in order to restore the original wall thickness.
Turbine blades which have already developed cracks during the production process may, for example, be made fit for use by build-up welding, so that scrap can be avoided in production.
However, at present, it is only with difficulty that the γ′-strengthened nickel-based superalloys can be welded with welding fillers of the same type by means of conventional welding methods. The reason for this is that micro-segregations, that is to say microscopic separations, of the molten material, must be avoided. Moreover, the welding process itself can lead to the generation of cracks in the welded region during subsequent heat treatments. These are caused by residual welding stresses due to plastic deformations during the heat input when welding.
In order to circumvent the difficult weldability of the γ′-hardened nickel-based superalloys, welding is often performed with ductile welding fillers, for instance with nickel-based alloys without γ′ hardening. One such typical nickel-based alloy without γ′ hardening is, for example, IN 625. The ductility of the filler that is not γ′ hardened allows the reduction of welding stresses due to plastic deformations during the first heat treatment after welding. However, the unhardened alloys have lower high-temperature resistance (both low tensile strength and low creep strength) in comparison with γ′-hardened nickel-based superalloys. Therefore, welding methods without ductile fillers are used with preference. These methods may be divided into two classes, methods in which overaging of the base material takes place to increase the ductility by means of coarsening of the γ′ phase and methods in which the welding process is carried out with a preheated substrate. Carrying out the welding process on a preheated substrate avoids the residual welding stresses by means of recovery during the welding process. A welding process with prior overaging is described, for example, in U.S. Pat. No. 6,120,624, a welding process carried out on a preheated workpiece is described, for example, in U.S. Pat. No. 5,319,179.
However, the two mentioned welding methods without ductile welding fillers likewise have disadvantages. For example, in the case of overaging carried out before the welding process, a corresponding heat treatment of the γ′-hardenable nickel-based superalloys is carried out before the welding, in order to bring about the overaging of the γ′ phase. The ductility of the base material is thereby increased significantly. This increase in the ductility makes it possible to weld the material at room temperature. Moreover, it can be cold-straightened. Furthermore, such a heat treatment makes it possible for nickel-based superalloys such as, for example, Rene 41 or Haynes 282 to be used as a welding filler. Although these form γ′phases in the microstructure, they do so only with a significantly smaller proportion of the volume than the typical γ′-containing nickel-based superalloys that are used nowadays for gas turbine hot-gas components, such as gas turbine blades (for example IN 738 LC, IN 939, Rene 80, IN 6203 DS, PWA 1483 SX, Alloy 247, etc.). Therefore, even when overaging is performed before the welding process, no full structural weldings take place.
If a preheating of the turbine blade is performed, the temperature difference and the resultant stress gradient between the weld point and the rest of the turbine blade is reduced, whereby the formation of welding cracks in components of nickel-based superalloys can be avoided. Such methods in which preheating of the turbine blade to temperatures between 900° C. and 1000° C. is performed by means of induction coils must, however, be carried out under shielding gas, which makes the welding process complicated and expensive. Moreover, as a result of lack of accessibility to the workpiece located in a shielding gas enclosure, this method cannot be carried out on all regions of the workpiece.